Getting started

I'm just about to get my first sensor readout to Home Assistant but something is wrong and I need your help.

I now have Home Assistant running on Raspberry Pi 3 and an Arduino Uno acting as a serial gateway flashed with the example gateway sketch. And one sensor node on Pro Mini with DHT22 attached and flashed with the example DHT sketch.

From examining the logs I can see that Home Assistant is able to connect to the gateway:2017-11-14 22:35:03 INFO (Thread-13) [mysensors.gateway_serial] Trying to connect to /dev/ttyACM0
2017-11-14 22:35:03 INFO (Thread-13) [mysensors.gateway_serial] /dev/ttyACM0 is open...
2017-11-14 22:35:03 INFO (Thread-13) [mysensors.gateway_serial] Connected to /dev/ttyACM0

Yes, I would recommend that. From personal experience computer usb powered unos are unstable. Make sure you have a capacitor close to the radio, as recommended in the radio guide on the mysensors site, too.

@martinhjelmare ok, I'll get a proper power supply. I have a capasitor welded to the radio module. The gateway has the model with the stick antenna.

I have tried how far the demo sensor node works (it has the std module with the antenna on the PCB) and it works anywhere in my 3 story house.
But when I moved it to a building next to my house it didn't work. The distance was maybe 40m.

Should I experiment with the setting #define MY_RF24_PA_LEVEL ?
What does this require from the HW?

You can of course try pa level max. It might help. As @gohan said, the pa/lna version would also help in this situation. You can also try directional antenna yagi to maximise power in the direction you want it to go (fairly cheap on ebay).

What is between the buildings? 40m of free space?

Also think about using the 5V power from the uno and going via a buck converter (or any linear or switching voltage regulator) down to 3.3v - the arduino 3.3v supply is notorious for being 'noisy'..

Adding a gateway means that your current system stays the same and is not changed in any way - so you have time to experiment with the new additions.

I have never used rfm69 so can't say on those, sorry.

I will say that you probably can't have 2 radios on one gw. But it might be possible.
With MyController you can have multiple gateways for the controller and that is what you will need.
As per above, I would just add another pro mini to act as a second gw for the longer range testing as that will not impact on your current setup.

You can't do it on an arduino. Since you are going to run a controller it could make sense having a raspberry. Running 2 separare instances it would not be a problem for HA as you would have to add the 2 gateways anyway like if they were on separate hw